Method and device for bonding a non-woven fibre produced by the air-lay method

ABSTRACT

The conventional method for production of a multiple-layer, non-woven fibre is by means of the air-lay method, with thermal bonding using bonding fibres. The same method can be applied to a composite non-woven fibre with an intermediate pulp layer. This method of bonding does not reduce the later pilling wear and hardly influences the inner composition of the layers of the composite. According to the invention, the outer non-woven fibre is preferably made from a bicomponent fibre and treated by hydrodynamic needling for bonding, such that not just the surface is bonded, but also the layers are bonded to each other.

[0001] WO 97/30223 makes known an air-lay method for producing a composite non-woven fabric, where a plurality of air-lay moulding heads of the type according to EP 0 032 772 are disposed one behind the other and produce a composite non-woven fabric from connecting fibres, then cellulose fibres and again connecting fibres. This type of production has the advantage of higher achievable speed compared to the formation of card non-woven fabric. For strengthening, a calender and a through-ventilation unit are connected and the object of these is to attach the connecting fibres to one another using heat.

[0002] This type of strengthening does not create a non-woven fabric which meets the conditions in practice. The individual layers have no cohesive strength, the non-woven fabric components delaminate and split. The surfaces of the non-woven fabric are also liable to wear and tear.

[0003] The object of the invention is to find a method, the fibres from which should make up this type of non-woven fabric for the air-lay method and subsequently to find how this fabric should be strengthened to prevent, amongst other things, delaminating.

[0004] Proceeding from a method, where for the strengthening of a non-woven fabric, which has been produced according to the air-lay method,—comprising two outer layers made of cut, thermally activatable fibres and one layer of cellulose fibres, such a super-absorbent pulp, which is disposed between these outer layers, through continuous forming of initially the lower layer, then overlaying with the pulp layer and finally the top layer—simply one thermal treatment is provided, it is provided according to the invention to form initially the two outer layers of the composite non-woven fabric from fibres which are from 8 to 10 mm maximum in length and then to subject the three-layered non-woven fabric to a hydrodynamic needling process for strengthening, that is to say also for the intimate bonding of the layers, which are laid solely according to the air-lay method. In an advantageous manner here, the multilayered composite non-woven fabric comprises two outer layers, which are formed according to the air-lay method from fusion adhesive fibres, such as more especially double-component fibres or and other fibres, such a high polymer fibres, between which layers is placed another layer of super-absorbent pulp, also produced solely according to the air-lay method, and whereby this composite non-woven fabric, which is produced solely according to the air-lay method, is subsequently strengthened by means of the hydrodynamic needling process.

[0005] This type of non-woven fabric composition for the surface layers and the type of strengthening by means of the water needling process guarantees a surface which is free of pilling and linting, but which, nevertheless, is porous to liquid and can also cover a stronger, absorbent pulp layer. The two outer layers of this type of non-woven fabric are intensively strengthened by the water needling process and are also bonded to the pulp layer, which is what the dreaded laminating of the fibre types after strengthening prevents. For sufficient strengthening of the surface layers, both the layers are preferably treated with the water needling process, that is to say the composite non-woven fabric is needled on both sides.

[0006] Solely hydrodynamic needling of a composite non-woven fabric is made known by U.S. Pat. No. 5,413,849. In this case however, the outer layers are deposited as card non-woven fabric, that is to say they are made from considerably longer staple fibres, and the central layer comprises resilient continuous fibres, which give the entire composite non-woven fabric a stability. The outer card non-woven fabric layers made of staple fibres can be replaced partially by the short pulp fibres, which can also be laid using the air-lay method. The same is valid in conjunction with WO 92/08834. In this case, a mixture of staple fibres and pulp fibres are water-needled, nothing being stipulated about the length of the fibres, however the fibres are not laid according to the air-lay method.

[0007] The fusion adhesive fibres should be treated for further bonding on each other and to each other by means of heat. The heat also influences the required surface finish of the non-woven fabric. In this respect, the double-component fibre is particularly advantageous because it remains preserved as original fibre during the heat treatment, whilst the normal fusion adhesive fibre can fuse and shrink, which is disadvantageous for the forming of the resistant surface. This measure of heating the fusion adhesive fibres is already known through U.S. Pat. No. 5,240,764, however not in conjunction with the double-component fibres. The heat treatment can be effected in the most varied way; the essential point is the fibres bonding well on the surface and transversely through the surface non-woven fabric.

[0008] A device of the type according to the invention is represented as an example in the drawing. Further inventive details can be explained by way of this example. In which:

[0009]FIG. 1 is the side view of a continuous system for the production and strengthening of a dry non-woven fabric, which has been laid according to the air-lay method, and

[0010]FIG. 2 is a system similar to FIG. 1 with no thermo bonding according to the lay method.

[0011] The non-woven fabric is initially formed from staple fibres and pulp. The staple fibres must be cut short enough for them to be laid according to the air-lay method, i.e. they are from 8 to 10 mm maximum in length. The moulding head 1 is then charged with the staple fibres for the base layer of the non-woven fabric. The staple fibres are such that they can be brought to adhesion under the influence of heat. More especially advantageous here are double-component fibres, which have the advantage that they do not shrink under the influence of heat, but remain preserved as fibre. Onto the base non-woven fabric formed with the moulding head 1 then comes the pulp layer with the moulding head 2, where applicable a thicker pulp layer again with another moulding head (not illustrated), and then the top surface layer made of staple fibres. This composite non-woven fabric 4 then has to be strengthened and up to now this has only been effected by means of thermo bonding.

[0012] According to the system in FIG. 1, the non-woven fabric 4 is first pre-strengthened by means of heat in the device 5. This can be effected by means of hot air (through-ventilation), but also with the application of infrared radiation or with calendering. This pre-strengthening measure has the advantage of obtaining better surface closure of the surface layers, which can prevent a greater loss of pulp during the subsequent needling process. The system shown in FIG. 2 does not have this pre-strengthening under heat. Therefore, depending on the fibre and other conditions, the pre-strengthening measure can be left out.

[0013] The composite non-woven fabric 4 is then acted upon by the hydrodynamic needling process. As, in this case, we are talking about a non-woven fabric with surface layers on both sides, it is expedient to execute the strengthening process with the water jets on both sides. This is why the needling arrangement is represented in the Figures with two drums 6 and 7, which is only to indicate that the non-woven fabric is to be conveyed in a meander-shaped manner about the drums so that it can be acted upon on both sides by the water jets. This water jet strengthening process not only influences the surface of the composite non-woven fabric, but also the deeper cohesive strength of the layers such that a subsequent delaminating of the individual non-woven fabric layers no longer occurs.

[0014] After the strengthening of the composite non-woven fabric and also the bonding of the layers with each other by means of the water jets, the non-woven fabric must be dried. For this purpose, a through-ventilation drier 8, such as a sieve drum drier, is advantageously advantageous. Finally, the non-woven fabric can be calibrated again by means of the arrangement 9, as is only represented in the system shown in FIG. 2.

[0015] This type of composite non-woven fabric is very economical because it can be produced at high speeds, such as, for example, 400-500 m/min. A very absorbent product of 40-100 g/m² can be produced, as the pulp layer can be thick. At the same time, the surface layers should be thin in relation to the pulp layer, for example 10 gm², whilst the pulp layer can be heavy at 60 g/m². Through a combination of thermo bonding and water needling, the surface is not only essentially free of linting and pilling, but the non-woven fabric is also strengthened through and through and this makes it difficult to separate the layers of the non-woven fabric. 

1. Method for strengthening a non-woven fabric, which has been produced according to the air-lay method, comprising two outer layers made of cut, thermally activatable fibres and one layer made of cellulose fibres, such as super-absorbent pulp, which is disposed between these outer layers, through continuous forming of initially the lower layer, then overlaying with the pulp layer and finally the top layer, characterised in that the two outer layers of the composite non-woven fabric are formed from fibres, which are from 8 to 10 mm maximum in length, and the three-layered non-woven fabric is subjected to a hydrodynamic needling process for the strengthening, that is to say also for the intimate bonding of the layers, which are laid solely according to the air-lay method.
 2. Method according to claim 1, characterised in that in addition to the strengthening by means of the hydrodynamic needling process, the composite non-woven fabric, which is formed solely according to the air-lay method, is treated by means of a thermal strengthening process.
 3. Method according to claim 2, characterised in that the term thermal strengthening includes the treatment of the non-woven fabric, which is formed solely according to the air-lay method, with every type of heat treatment, namely heat treatment, for example, with hot air, more especially through-ventilation, with infra-red, with calendering.
 4. Method according to claim 2 or 3, characterised in that the composite non-woven fabric, which is formed solely according to the air-lay method, is thermally treated before and/or after the water needling process for strengthening.
 5. Method according to one of the claims 2-4, characterised in that the thermal strengthening together with the drying of the non-woven fabric, which is formed solely according to the air-lay method, is executed after the water needling process.
 6. Method according to one of claims 2-5, characterised in that at the end of the strengthening process, the composite non-woven fabric, which is formed solely according to the air-lay method, is calibrated again.
 7. Method according to one of claims 1-6, characterised in that the hydrodynamic treatment is executed on both sides of the composite non-woven fabric, which is formed solely according to the air-lay method.
 8. Device for executing the method in accordance with one of claims 1-7, comprising the following disposed one after the other in a line a) at least three moulding heads working according to the air-lay method, one for the first layer made of thermally activatable fibres with a fibre length from 8 to 10 mm maximum, one for the pulp and one for the top surface layer again for the thermally activatable fibres with a fibre length of from 8 to 10 mm maximum, and a b) non-woven fabric strengthening unit, which comprises a hydrodynamic needling device (6, 7) and a drier (8).
 9. Device according to claim 8, characterised in that upstream of the device for the hydrodynamic needling of a composite non-woven fabric, which is formed solely according to the air-lay method, c) is disposed a device (5) for the thermal strengthening.
 10. Device according to claim 8 or 9, characterised in that downstream of the device for the hydrodynamic needling of a composite non-woven fabric, which is formed solely according to the air-lay method, d) is disposed a device (8) for the thermal strengthening.
 11. Device according to claim 10, characterised in that the device (8) for the thermal strengthening of a composite non-woven fabric, which is formed solely according to the air-lay method, is provided after the hydrodynamic needling initially for the drying of the non-woven fabric.
 12. Device according to one of claims 8-11, characterised in that a calibrating device (9) is provided as the last unit for treating the composite non-woven fabric, which is formed solely according to the air-lay method.
 13. Multi-layered composite non-woven fabric comprising two outer layers, which are formed in accordance with the air-lay method from fusion adhesive fibres, which are from 8 to 10 mm maximum in length, such as more especially double component fibres or and other fibres, such as high polymer fibres, between which layers is laid a further layer made of super-absorbent pulp, which is also produced according to the air-lay method, wherein this composite non-woven fabric, which is formed solely according to the air-lay method, is strengthened by means of the hydrodynamic needling. 